Heart rates and how/why they fluctuate as a person's activity level changes.

The heart rates is fluctuated by a person's change in physical activities. This is because, with the increase in the physical activity, the demand of oxygen in the blood and in body increases. Since more energy is releases, more oxygen is needed. To supply more oxygen to the blood, heart needs to pump more blood in and out. this, leads to an increase in the heart rate.